A cathode for an electron gun of a cathode-ray tube in general comprises a cap on which a material intended to emit an electron beam is placed, a cylindrical-shaped cathode skirt forming, with the cap, a unitary assembly, a heater filament inserted in the skirt, the filament comprising in general a spiral part placed close to the cap and the leads for connection to the power supply circuit, this connection being made via the skirt orifice situated opposite the cap; the filament leads are welded to rigid yokes secured to the structure of the gun through electrically non-conducting parts made, for example, of glass. The cathode itself is held in place in the lower part of the gun using a sleeve secured, for example by welding, to the end of the skirt opposite the cap. Such a structure is, for example, described in U.S. Pat. No. 4,403,169.
In a gun of this type, part of the power provided by the filament to take the emitting part to its operating temperature, is lost by radiation at the rear opening of the cathode skirt. Furthermore, a lot of power is lost through the cathode support means in the gun, such as the sleeve mentioned above. In order to improve the thermal efficiency of the cathode, U.S. Pat. No. 5,013,965 describes the use a cylindrical cathode skirt having the particular feature of being closed at the end opposite the cap. This configuration improves the thermal efficiency of known cathodes by heat conduction from the skirt to the emitting part, but in an insufficient manner and in particular it does not especially speed up the cathode startup time, an important characteristic for rapidly obtaining an image on a television screen.
A cathode according to the prior art, as illustrated in FIG. 1, comprises an emitting part 1 in the form of a layer of emitting material for an oxide cathode or of a pellet impregnated with emitting materials in the case of a so-called impregnated cathode. The emitting material is supported by a cathode cap 2, placed at one of the ends of a cylindrical skirt 3; the skirt 3 lies in a direction Z perpendicular to the emitting surface of 1; the skirt 3 is closed at its end 4 opposite the cap, so as to form a closed cylinder in which the cathode filament 5 is enclosed; the filament leads pass through the cathode skirt via orifices 9 made in the end 4 opposite the cap. The cathode is supported in the structure of the gun by a sleeve 7 connected in a conventional manner to the other parts of the electron gun.
However, this type of structure has two major drawbacks:                its length along the main axis Z is large and contributes to extending the length of the electron gun which incorporates it and consequently, the depth of the tube fitted with such a gun; and        the thermal efficiency of such a cathode is not optimized. A lot of energy is lost in the skirt and in the means for connecting the skirt to the other parts of the gun.        